The present invention relates to DX circuits in general, and in particular to an improved DX circuit that may be connected to reference and signaling leads of a DX signaling link, without need to balance the internal impedance of the DX circuit against that of the external signaling link.
One common method for trunk signaling, between central office or PBX switches, uses E & M signaling leads, which allows for simultaneous two-way or duplex signaling between offices. The E & M signaling interface provides a standard interface between a variety of switches, for example between trunks and signaling units or between signaling units themselves. E & M signaling conveys supervisory and dial address information to a switching device using two control leads (E & M), and both ends of the circuit use compatible signaling units.
E & M signaling ranges are relatively short, so to extend this type of duplex signaling over any reasonable distance, it is necessary to use DX circuits. Conventionally, a DX signaling circuit uses a series of relay windings to convert E & M signaling to DX signaling, and two separate leads, a signaling lead and a reference lead, to indicate the E & M states on either side of the circuit. DX circuits can be used on 2 Wire or 4 Wire facilities, and are installed on trunk lines or carrier facilities that connect PBXs to central offices or central offices to central offices. Two DX circuits are used in any application to convert E & M signaling to DX signaling at each end of the circuit. In cases where DX circuit outputs are connected directly to line amplifiers, they are connected via center-tapped simplex leads. In 2 Wire applications, they are connected via A and B leads.
A pair of conventional DX signaling units is a balanced bridge. As such, each must be resistively balanced against the resistance of the signaling loop, plus the internal resistance of the unit itself. In 2 Wire applications, signaling loop[resistance is simply the resistance of the metallic facility between the DX signaling units. In 4 Wire applications, where signaling takes place over simplex leads of transmit and receive pairs, signaling loop resistance is equal to one-half of the loop resistance of either pair, i.e., the simplex loop resistance of the transmit and receive pair. The DX signaling unit therefore has an internal resistive balance network, which may be adjusted to provide a selected amount of resistance across the unit's outputs.
Conventional DX circuits must also be balanced for facility capacitance. No specific formula exists for calculating the amount of capacitance required to properly balance the circuit. The amount depends upon a variety of factors. For example, little capacitive balance is required in most 4 Wire DX circuits because the signaling pairs are separated by relatively substantial distances, and are therefore coupled by very little mutual capacitance. Numerous other factors, including cable gauge and splicing format, also affect DX signaling link capacitance. The cumulative effect of these factors makes prediction of the required amount of balancing capacitance difficult. A trial-and-error procedure is therefore necessary to achieve proper capacitive balance by means of capacitors that may be switched into the DX circuit balance network.
Unless a DX signaling unit's internal impedance is balanced against that of the external DX signaling link, the unit may not operate properly. Proper DX balance ensures optimum performance of the DX signaling unit for a specific length of a DX signaling link, and also minimizes pulse distortion. Unfortunately, proper DX balance is often very difficult to achieve.